Squeezing more from less: How to design smart gas and water meters for the ultimate in energy efficiency

It is important to identify the dominant power consumption tasks in the metering application. In our gas or water meter example, there are two primary tasks:

Check the state of a reed switch 20 times per second to calculate flow.

Formulate a radio data packet every 15 seconds, and pass that data to the radio transmitter for broadcast.

In many metering applications, a device called a register encoder records the flow of natural gas or water. To the metering system, this can appear electrically as a series of switch closure events or pulses. In a traditional system, the CPU must wake up and sample the state of an I/O pin to determine if the switch is open or closed. If it is a physical reed switch, additional CPU bandwidth is needed to de-bounce the switch as well as manage pull-up resistors to guarantee it is a valid pulse as well as to minimize the current drain through the closed switch. Performing this function in software, even in the most optimized system, can consume well over 1 µA.

A better approach is to use a dedicated input capture timer that can operate autonomously while the device is in sleep mode. This technique has a number of advantages over a software-based approach. Primarily, the switch closures can be accumulated in a hardware register requiring little if any CPU intervention.

Additionally, features, such as switch de-bounce, pull-up resistor management and self-calibration, can be integrated directly in the hardware. With two timer inputs, quadrature decode functionality can be supported to determine flow direction. This provides the capability of back-flow detection as well as an anti-tamper provision. A dedicated low-power input capture timer can consume as little as 400 nA at 3.6 V even with a sampling rate as high as 500 Hz. This is a significant improvement over performing this function in software.

When a CPU is running, it typically fetches instructions from non-volatile memory (e.g. flash). It is not uncommon for 40 percent of the active mode current to be attributed to flash access reads. For this reason, any time we are able to move data using dedicated hardware peripherals instead of the CPU, we can save power.

When preparing a message for RF transmission, the data must be manipulated several times. For example, let’s assume you have a 20 byte message payload that needs to be transmitted from the meter to the collector. Initially, these 20 bytes reside in SRAM. However, the data may include private customer information and, therefore, must be encrypted. Afterwards, a cyclic redundancy check (CRC) is computed and appended on the end of the encrypted message. Finally, the entire message will be encoded (e.g., Manchester, 3:6, etc.) before it is serially passed through the serial peripheral interface (SPI) to the radio transceiver. All of these functions can be performed in software using the CPU. However, it is much more efficient to have dedicated hardware, such as a dedicated packet processing engine (DPPE) as shown in Figure 4, perform these tasks.

Click on image to enlarge.

Fig 4: Processing time and power savings achieved with DPPE hardware block.

Using a DPPE not only reduces the time needed to perform the functions, but it also reduces the current consumption during that time since flash memory is not being accessed. The net result can be up to a 90 percent power reduction during active mode. With these improvements, we are able to exceed the savings target for active mode, making it only 6 percent of the overall budget as shown in figure 5.

Fig 5: Smart meter power reduction results achieved using DPPE.

After applying all three of the power saving techniques, we were able to successfully raise the TX power budget to 70 percent through a complete subsidy of savings from RX, sleep, and active modes. In other words, we met the overall design objectives of increasing the TX reliability without using a larger battery or reducing the original target life.

This example demonstrates how power savings could be applied to redistribute the overall budget in a smart meter application. However, power savings can be valuable in a number of other ways. One obvious example is the ability to use a smaller, lower-cost battery. Another benefit may be to increase the battery’s target life using the same battery. A less obvious benefit is greater design margin and reduced warranty liability. Consider a scenario in which a meter manufacturer produces millions of units per year, each with a 20-year service warranty. If meters begin to fail after 15 years because of excessive power consumption, the potential liability to the supplier can involve tens of millions of meters. Ultimately, additional design margin provides peace of mind for engineers and investors alike.

Keith Odland is Director of Marketing, Microcontrollers, at Silicon Labs.
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Change in design with circuits really helpful to increase the effeciency of different units. I have heard that we can save some money by installaing energy effeicent applinaces to our home. I was just thinking it was the myth. Two years ago i installated Rinnai tankless water warmer with new design and uopdated technology it make me amazed that almost about 20 to $40 has been saved every month due to it and its performance also better then the proviuos one.

@tcyr, there is few hundred patents about harvesting energy from flow of fluid, maybe we can not suggest this openly without violating other(s) patent rights, by product itself I have seen none, most likely the patent holders are not into manufacturing and manufacturer unable to pay patent holder cost. (I assume only, as this type of gridlock happened in other sector too). Again this article was about saving energy for design that uses battery and needed more TX power, I think the author address is clearly.

@tcyr: I agree with you partially, harvesting power from the fluid flow would be nice to implement in a self-powered gas meter with battery backup (as is the case with many energy monitoring devices for electricity that scavenge the line power). The author does cover topics that aren't in a typical Enginering 101, perhaps 201 or higher!
MP Divakar

Sorry, but this is Engineering 101. I was expecting a more original idea given our advances in power storage and "green" technologies.
How about a design for a meter that uses a micro-turbine which generates power locally based on the flow of gas/water through the meter and charges an Ultra-Cap?!? The loss in service pressure due to the energy extraction would be transparent to the end user (from what, 100psi?, down to maybe 90psi? the final regulator on my water heater regulates it down to 25psi anyway!). A teflon based magnetically coupled micro-turbine would be safe for use with natural gas since no spark hazard would exist, and there is a huge amount of potential energy available in a pressurized gas line! The size of the micro-turbine would be based on the max operating current requirements of the meter and scaled to support operation even during "low flow" periods. In this mode the battery would exist purely for backup power so a significantly smaller battery could be used (or possibly a rechargeable battery?). The power generating capability of the micro-turbine could also be sized in order to reduce the necessity for the extra design effort required just to pinch micro-amps from every area of circuit operation, as this article goes in great depth to describe. The question then becomes about guaranteeing the minimum life expectancy of a micro-turbine over that of a chemical battery, which since there would be no gears or mechanical parts to wear out should not be a problem.

This is a very good technical article. It shows good engineering practices. It impresses me that it talks about the power consumption due to de-bounce switches software routine. Now I know that an alternative to that is to implement it with hardware. Anyone has a link to where I can find such kind of information?

every 15 seconds transmitting the data by RF seems to be expensive. The data can be stored in the unit and the network station can send a query every 24 hours and then the meter can reply. This will make the whole system more economical.